Quantization parameter derivation for cross-channel residual encoding and decoding

Abstract

There are disclosed various methods, apparatuses and computer program products for video decoding or encoding. In some embodiments for decoding a block that contains quantized residual coefficients based on at least two color channels of a video presentation or an image is received. A first quantization parameter and an offset defining a relationship between the first quantization parameter and a second quantization parameter for the block are obtained. The value of the second quantization parameter is determined by using the first quantization parameter and the offset. The residual samples are reconstructed by dequantizing the quantized residual coefficients using the second quantization parameter, if the block has been encoded by using a cross-channel residual coding mode.

The invention claimed is: 
     
       1. A method comprising:
 obtaining a first quantization parameter derived for a luminance channel of a video presentation or an image, the first quantization parameter configured to be used as a reference to determine a value of a second quantization parameter; 
 receiving a block that contains quantized residual coefficients of a first chrominance channel and a second chrominance channel of the video presentation or the image; 
 obtaining an offset; 
 determining whether the block has been encoded using a cross-channel residual coding mode; 
 determining the value of the second quantization parameter using the first quantization parameter derived for the luminance channel and the offset, when the block has been encoded using the cross-channel residual coding mode, the second quantization parameter configured to be used to jointly dequantize the residual coefficients of the first chrominance channel and the second chrominance channel; and 
 reconstructing residual samples for the first chrominance channel and the second chrominance channel with dequantizing the quantized residual coefficients of the first chrominance channel and the second chrominance channel jointly using the value of the second quantization parameter determined using the first quantization parameter derived for the luminance channel and the offset, when the block has been encoded using a cross-channel residual coding mode. 
 
     
     
       2. The method according to  claim 1  further comprising:
 determining whether a value of the first quantization parameter is within a range; 
 adjusting the value of the first quantization parameter when the value of the first quantization parameter is not within the range; and 
 determining the value of the second quantization parameter using the value of the first quantization parameter. 
 
     
     
       3. The method according to  claim 1  further comprising:
 obtaining one or more limiting criteria, wherein the one or more limiting criteria comprises at least one of: the second quantization parameter not being smaller than a minimum value, the second quantization parameter not being greater than a maximum value, or the second quantization parameter not being outside a value range; and 
 limiting the value of the second quantization parameter to fulfil the one or more limiting criteria. 
 
     
     
       4. The method according to  claim 1  further comprising:
 determining another offset based on a bit-depth of the video presentation or the image; and 
 determining the value of the second quantization parameter using the another offset. 
 
     
     
       5. The method of  claim 1 , further comprising:
 providing an indication that the block is to be encoded using the cross-channel residual coding mode. 
 
     
     
       6. The method of  claim 1 , further comprising:
 determining that the block is to be encoded using a cross-channel residual coding mode when there is a lack of an indication that the block is to be encoded using the cross-channel residual coding mode. 
 
     
     
       7. The method of  claim 1 , wherein:
 the offset is indicated in a picture parameter set; and 
 a second offset within a tile group header is used to determine the value of the second quantization parameter. 
 
     
     
       8. The method of  claim 1 , wherein a value of the offset is a negative integer or zero. 
     
     
       9. The method of  claim 1 , further comprising:
 determining whether the offset is common for multiple blocks or comprises a predetermined value; 
 wherein the offset is not determined for blocks separately, when the offset is common for multiple blocks or comprises a predetermined value. 
 
     
     
       10. The method of  claim 1 , further comprising:
 determining the value of the second quantization parameter when the value of the first quantization parameter changes. 
 
     
     
       11. An apparatus comprising:
 at least one processor; and 
 at least one non-transitory memory including computer program code; 
 the at least one non-transitory memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: 
 obtain a first quantization parameter derived for a luminance channel of a video presentation or an image, the first quantization parameter configured to be used as a reference to determine a value of a second quantization parameter; 
 receive a block that contains quantized residual coefficients of a first chrominance channel and a second chrominance channel of the video presentation or the image; 
 obtain an offset; 
 determine whether the block has been encoded using a cross-channel residual coding mode; 
 determine the value of the second quantization parameter using the first quantization parameter derived for the luminance channel and the offset, when the block has been encoded using the cross-channel residual coding mode, the second quantization parameter configured to be used to jointly dequantize the residual coefficients of the first chrominance channel and the second chrominance channel; and 
 reconstruct residual samples for the first chrominance channel and the second chrominance channel with dequantizing the quantized residual coefficients of the first chrominance channel and the second chrominance channel jointly using the value of the second quantization parameter determined using the first quantization parameter derived for the luminance channel and the offset, when the block has been encoded using a cross-channel residual coding mode. 
 
     
     
       12. The apparatus according to  claim 11 , wherein the apparatus is further caused to:
 determine whether a value of the first quantization parameter is within a range; 
 adjust the value of the first quantization parameter when the value of the first quantization parameter is not within the range; and 
 determine the value of the second quantization parameter using the value of the first quantization parameter. 
 
     
     
       13. The apparatus according to  claim 11 , wherein the apparatus is further caused to:
 obtain one or more limiting criteria, wherein the one or more limiting criteria comprises at least one of: the second quantization parameter not being smaller than a minimum value, the second quantization parameter not being greater than a maximum value, or the second quantization parameter not being outside a value range; and 
 limit the value of the second quantization parameter to fulfil the one or more limiting criteria. 
 
     
     
       14. The apparatus according to  claim 11 , wherein the apparatus is further caused to:
 determine another offset based on a bit-depth of the video presentation or the image; and 
 determine the value of the second quantization parameter using the another offset. 
 
     
     
       15. The apparatus according to  claim 11 , wherein the apparatus is further caused to:
 use information of the luminance channel; and 
 use information of the channel which has a highest spatial resolution. 
 
     
     
       16. A method comprising:
 determining a first quantization parameter derived for a luminance channel of a video presentation or an image, the first quantization parameter configured to be used as a reference to determine a value of a second quantization parameter, 
 determining an offset; 
 determining whether a block of residual samples for a first chrominance channel and a second chrominance channel of the video presentation or the image has been encoded using a cross-channel residual coding mode; 
 determining the value of the second quantization parameter using the first quantization parameter derived for the luminance channel and the offset, when the block has been encoded using the cross-channel residual coding mode, the second quantization parameter configured to be used to jointly quantize the residual coefficients of the first chrominance channel and the second chrominance channel; and 
 quantizing the residual samples jointly for the first chrominance channel and the second chrominance channel of the video presentation or the image to obtain jointly quantized residual coefficients of the first chrominance channel and the second chrominance channel using the value of the second quantization parameter determined using the first quantization parameter derived for the luminance channel and the offset, when the block has been encoded using the cross-channel residual coding mode. 
 
     
     
       17. An apparatus comprising
 at least one processor; and 
 at least one non-transitory memory including computer program code; 
 the at least one non-transitory memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: 
 determine a first quantization parameter derived for a luminance channel of a video presentation or an image, the first quantization parameter configured to be used as a reference to determine a value of a second quantization parameter, 
 determine an offset; 
 determine whether a block of residual samples for a first chrominance channel and a second chrominance channel of the video presentation or the image has been encoded using a cross-channel residual coding mode; 
 determine the value of the second quantization parameter using the first quantization parameter derived for the luminance channel and the offset, when the block has been encoded using the cross-channel residual coding mode, the second quantization parameter configured to be used to jointly quantize the residual coefficients of the first chrominance channel and the second chrominance channel; and 
 quantize the residual samples jointly for the first chrominance channel and the second chrominance channel of the video presentation or the image to obtain jointly quantized residual coefficients of the first chrominance channel and the second chrominance channel using the value of the second quantization parameter determined using the first quantization parameter derived for the luminance channel and the offset, when the block has been encoded using the cross-channel residual coding mode. 
 
     
     
       18. The apparatus according to  claim 17 , wherein the apparatus is further caused to:
 determine whether a value of the first quantization parameter is within a range; 
 adjust the value of the first quantization parameter when the value of the first quantization parameter is not within the range; and 
 determine the value of the second quantization parameter using the value of the first quantization parameter. 
 
     
     
       19. The apparatus according to  claim 17 , wherein the apparatus is further caused to:
 obtain one or more limiting criteria, wherein the one or more limiting criteria comprises at least one of: the second quantization parameter not being smaller than a minimum value, the second quantization parameter not being greater than a maximum value, or the second quantization parameter not being outside a value range; and 
 limit the value of the second quantization parameter to fulfil the one or more limiting criteria. 
 
     
     
       20. The apparatus according to  claim 17 , wherein the apparatus is further configured to:
 use information of the luminance channel; and 
 use information of a channel which has a highest spatial resolution.